Collagens are an ubiquitous family of extracellular proteins which are expressed as proproteins that undergo extensive posttranslational modifications. These include hydroxylation of lysine and proline, glycosylation of asparagine and hydroxylysine (the first restricted to the portion of the propeptide that is deleted in the final form), truncation of N- and C-termini, assembly into stable hydrogen-bonded triple helices, and crosslinking through disulfide and hydroxylysine residues. Although the cDNA sequences of many collagens have been determined and the general types of posttranslational modifications have been defined, little has been reported regarding complete structural determinations of the expressed proteins. The first step in such a study is the measurement of the molecular weights of the individual alpha-chains (around 100 kDa) and the intact triple helices (250-450 kDa). We have found that infrared and ultraviolet matrix-assisted laser desorption/ionization ca n be used for this purpose for accurate determination of both forms and that, under suitable experimental conditions, the forms may be interconverted. We find that IR-MALDI is far superior to UV-MALDI for this class of compounds. Various types of collagens from different species and tissues have been examined using the method. It is now also being applied to the study of other proteins with collagen-like domains.